Microfabricated flexible ground reaction sensor cluster for navigation in GPS-denied environments

Abstract

Embodiments are directed to a ground reaction sensor cluster (GRSC) and to methods for precisely determining zero velocity points and bearing changes using a GRSC and for navigating using a GRSC and an inertial motion unit (IMU) in a global positioning satellite (GPS)-denied environment. The GRSC device itself includes an array of capacitive pressure and shear sensors. The array includes multiple flexible capacitive sensor cells that detect changes in capacitance in response to a footstep. Each cell of the array includes multiple overlapping, fingered capacitors that detect pressure and shear force by determining the change in capacitance in each fingered capacitor. The GRSC device also includes a multiplexing receiver that receives the capacitance inputs from each of the capacitive sensor cells. The multiplexing receiver and other electronic elements further process the received capacitance inputs to determine, based on the pressure and shear forces, the direction and bearing of the footstep.

We claim: 
     
       1. A ground reaction sensor cluster (GRSC) comprising the following:
 an array of capacitive pressure and shear sensors, the array comprising a plurality of flexible capacitive sensor cells that detect changes in capacitance in response to a footstep, each cell of the array comprising a plurality of overlapping, fingered capacitors that detect pressure and shear force by determining the change in capacitance in each fingered capacitor, wherein the array of sensors is distributed along a contact portion of a shoe; 
 a multiplexing receiver that receives the capacitance inputs from each of the plurality of capacitive sensor cells, wherein the multiplexing receiver is communicatively connected to one or more electronic elements that further process the received capacitance inputs to determine, based on the detected pressure and shear forces, the direction and bearing of the footstep. 
 
     
     
       2. The ground reaction sensor cluster of  claim 1 , wherein the GRSC's array of flexible capacitive sensor cells comprises an array of flexible top, fingered electrodes and bottom, fingered electrodes which are sensitive to both normal and shear strain. 
     
     
       3. The ground reaction sensor cluster of  claim 2 , wherein one or more of the top, fingered electrodes are placed over the flexible, bottom electrodes, the cells comprising an elastic dielectric material. 
     
     
       4. The ground reaction sensor cluster of  claim 3 , wherein upon receiving normal force over the top, fingered electrode, the elastic dielectric material of the cells is compressed, increasing capacitance between the top, fingered electrode and the bottom, fingered electrodes equally, and upon receiving a combination of normal and shear force over the top, fingered electrode, the elastic dielectric material of the cells is compressed unevenly, shifting the top, fingered electrode and changing capacitance between the top, fingered electrode and each of the bottom, fingered electrodes in an uneven manner. 
     
     
       5. The ground reaction sensor cluster of  claim 4 , wherein the normal force is measured by the amount of equal increase in capacitance between the top, fingered electrode and the bottom, fingered electrodes, and the shear force is measured by the amount of changing capacitance between the top, fingered electrode and each of the bottom, fingered electrodes. 
     
     
       6. The ground reaction sensor cluster of  claim 1 , further comprising a communicatively connected inertial motion unit (IMU). 
     
     
       7. The ground reaction sensor cluster of  claim 6 , wherein the capacitance inputs from the array of capacitive pressure and shear sensors are combined with acceleration inputs from the IMU to determine a zero point at which a user's shoe is at zero velocity based on the detected pressure and shear sensor measurements. 
     
     
       8. The ground reaction sensor cluster of  claim 7 , wherein the shear measurements measure slippage and a corresponding intended direction of shoe rotation, independent of angular velocity magnitude. 
     
     
       9. The ground reaction sensor cluster of  claim 1 , wherein two overlapping, fingered capacitors are placed orthogonal to each other in each cell of the capacitive pressure and shear sensors. 
     
     
       10. The ground reaction sensor cluster of  claim 1 , wherein the multiplexing receiver is communicatively connected to a capacitance-to-voltage (C/V) converter that samples the capacitance inputs from the capacitive sensor cells, followed by an analog-to-digital converter (ADC) sampled at predefined number of samples per second, a digital control unit, and driving circuitry that allows further processing based on the capacitive inputs. 
     
     
       11. The ground reaction sensor cluster of  claim 1 , wherein the GRSC sensor array is manufactured using the following steps:
 patterning one or more bottom electrodes for each flexible capacitive sensor cell over photoresist placed on a film layer; 
 depositing a first conductive layer for the bottom electrodes by electronic-beam evaporator over the patterned photoresist; 
 applying an adhesion promoter to facilitate application of an organic silicon layer; 
 spin coating the organic silicon layer on the adhesion promoter; 
 curing the organic silicon layer; 
 depositing a Parylene layer over the organic silicon layer after exposing the organic silicon layer to a Parylene adhesion promoter; 
 patterning top electrodes for each cell over the Parylene layer; and 
 depositing a second conductive layer for the top electrodes by electronic-beam evaporator over the patterned Parylene layer. 
 
     
     
       12. A method for precisely determining zero velocity points and bearing changes using a ground reaction sensor cluster (GRSC), the method comprising:
 detecting a user's footstep using an array of pressure and shear sensors distributed along a contact portion of a user's shoe, wherein pressure sensing elements of the GRSC measure the distribution of ground reaction and shear sensing elements of the GRSC measure lateral forces and torque; 
 determining a zero point at which the user's shoe is at zero velocity based on the detected pressure and shear sensor measurements; 
 determining, based on the lateral force measured by the shear sensor at the zero point, the degree to which the user's bearing has changed; and 
 sending the bearing changes to an internal motion unit (IMU) communicatively attached to the GRSC, wherein the IMU uses the determined bearing change to refine its calculated zero point and velocity determinations. 
 
     
     
       13. The method of  claim 12 , further comprising using the refined zero point and velocity determinations to determine which direction the user's foot is heading. 
     
     
       14. The method of  claim 13 , further comprising providing navigation indications to the user, indicating where the user has been and the direction the user is currently going. 
     
     
       15. The method of  claim 14 , further comprising sending the navigation indications to a global positioning satellite (GPS)-enabled device, wherein the GPS-enabled device uses the indications to determine the user's location without being connected to the GPS satellites. 
     
     
       16. The method of  claim 12 , wherein the GRSC's array of flexible capacitive sensor cells comprises an array of flexible top, fingered electrodes and bottom, fingered electrodes which are sensitive to both normal and shear strain. 
     
     
       17. The method of  claim 16 , wherein one or more of the top, fingered electrodes are placed over the flexible, bottom electrodes, the cells comprising an elastic dielectric material. 
     
     
       18. The method of  claim 17 , wherein upon receiving normal force over the top, fingered electrode, the elastic dielectric material of the cells is compressed, increasing capacitance between the top, fingered central square and the bottom, fingered electrodes equally, and upon receiving a combination of normal and shear force over the top, fingered electrode, the elastic dielectric material of the cells is compressed unevenly, shifting the top, fingered electrode and changing capacitance between the top, fingered electrode and each of the bottom, fingered electrodes in an uneven manner. 
     
     
       19. A method for navigating using a ground reaction sensor cluster (GRSC) and an inertial motion unit (IMU) in a global positioning satellite (GPS)-denied environment, the method comprising:
 determining that a user has passed into an environment where a GPS device used by the user is no longer receiving GPS signals, wherein the GPS device is communicatively connected to a GRSC and an IMU; 
 detecting the user's footsteps using the GRSC, the GRSC comprising an array of pressure and shear sensors distributed along a contact portion of the user's shoe, wherein pressure sensing elements of the GRSC measure the distribution of ground reaction and shear sensing elements of the GRSC measure lateral forces and torque; 
 determining a zero point at which the user's shoe is at zero velocity based on the GRSC's detected pressure and shear sensor measurements; 
 determining, based on the lateral force measured by the shear sensor at the zero point, the degree to which the user's bearing has changed; 
 sending the bearing changes to an internal motion unit (IMU) communicatively attached to the GRSC, wherein the IMU uses the determined bearing change to refine its calculated zero point and velocity determinations; and 
 determining the user's direction and velocity using the measurements of the GRSC and the IMU to navigate the user until the user's GPS device is back in range of GPS satellites. 
 
     
     
       20. The method of  claim 19 , further comprising:
 receiving a GPS signal at the GPS device indicating that the GPS device is connected to a sufficient number of GPS satellites; and 
 using the measurements of the GRSC and the IMU to provide the GPS device with incremental updates regarding the user's continued movements.